[PATCH] PR libstdc++/87431 re-adjust never-valueless optimizations

[Ville Voutilainen]

On Wed, 3 Apr 2019 at 23:32, Antony Polukhin <antoshkka@gmail.com> wrote:
>
> Looks good. Covers most of the use cases.
>
> Please consider adding filesystem::path, pair, tuple, string_view?, error_code, list, deque (myabe all the other containers), optional, variant itself (for cases when variant holds another variant).
>
> It would be very useful for the _Never_valueless_alt to return true for aggregates that hold only never valueles types. Not sure that there is a compiler builtin for getting all the aggregates types, but it could be implemented via metaprogramming.

What metaprogram is capable of that?

> When is the ABI freeze for the C++17 additions?

This approach concerns me; it's a "get it right at the first chance
and then never change it", which is.. ..awkward. But I don't know
whether there's any
way around that.